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Thursday, June 07, 2012
Martial arts films' jaw-dropping fight scenes may leave you wondering if the laws of physics apply to Bruce Lee or Jet Li. Quite the opposite is true in real life, and an entire field of research specializes in understanding the biomechanics behind martial arts moves.

The martial art and Olympic sport of Judo – meaning "gentle way" – pits two competitors against each other in a battle of throwing, takedowns, and grappling maneuvers. Judo's throws rank among its best studied moves, where force and impulse (both the physical and mental property) play a key role. In preparation for the upcoming summer Olympic Games in London, here's a look into some of the physics behind Judo experts' impressive throwing skills. 

Judo principle explained by science

To measure the forces and impulse – the change in momentum – researchers have typically relied on a set of video cameras focused on Judo practitioners in a controlled setting. In one of the most widely-cited studies (PDF) of this sort from 2006, four black belt throwers (called tori) and one "faller" (called uke) were filmed performing a variety of standard Judo throws.

With this method, the researchers hoped to better understand which throws worked best for certain individuals. They measured impulse measurements for each participants' center of mass while undergoing three different kinds of throws.

Harai-goshi (Sweeping loin throw) Hip Technique 

Average Force: 158.9 Newtons over 0.63 seconds
Average Impulse: 100.1 Newton-seconds

Harai-goshi (Sweeping loin throw): This move created one of the largest impulses on the "fallers" body. Consequently, the researchers behind the study have labeled it a "power move" most suitable for larger individuals. For this move, the high-momentum thrower collides with the faller and abruptly loses momentum, leading to a relatively high impulse.


Seoi-nage (Shoulder Throw) Hand Technique

Average Force: 120.4 Newtons over 0.74 seconds
Average Impulse: 89.0 Newton-seconds

Seoi-nage (Shoulder throw): The shoulder throw had the lowest impulse of the three moves because the thrower maintains more of his momentum throughout the move. When there's a smaller change in momentum, the impulse is lower. The authors concluded that this move suited a shorter, speedier and nimbler thrower who can fit under their opponent.


Osoto-gari (Major outer reaping throw) Foot & Leg Technique

Average Force: 156.3 Newtons over 0.73 seconds
Average Impulse: 113.0 Newton-seconds

Together with the hip throw [Harai-goshi], and the leg throw [Osoto-gari] these techniques generally belong to the class of power moves suited for large, powerful throwers.
Note: This is not to say that Judoka that are smaller can't use [Harai-goshi] or [Osoto-gari] effectively against larger opponents, the understanding of action-reaction fighting while applying the principles of off-balancing [Kuzushi], fitting your body into your technique [Tsukuri], and completion of throw [Kake] are essential to throwing someone who is larger and or stronger, this is the principle of "Seiryoku zenyo" (Maximum efficiency) - Judo is based on the principle of a combatant using the motion and momentum of the opponent in order to apply his own power in an optimal manner, thereby generating even more power. 

                  Japanology - Judo 

All of the data for the three throws came from this Journal of Sports Science and Medicine research paper (PDF) by Rodney T. Imamura, Alan Hreljac, Rafael F. Escamilla and W. Brent Edwards of [Sacramento State University]. Check out the research for more about the biomechanics behind these throws, and make sure to tune in for some Judo this summer in London.
KODOKAN INSTRUCTIONAL - [Harai-goshi], [Seoi-nage], and [Osoto-gari]

Harai goshi

Seoi nage

O soto gari

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